22. 括号生成

22. 括号生成

回溯法

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> ans = new ArrayList<>();
        if (n <= 0)
            return ans;
        generate(new StringBuilder(), 0, 0, n, ans);
        return ans;
    }
    private void generate(StringBuilder currString, int left, int right, int max, List<String> ans) {
        if (currString.length() == max * 2) {
            ans.add(currString.toString());
            return ;
        }
        if (left < max) { // 能够继续添加左括号
            currString.append('(');
            generate(currString, left + 1, right, max, ans);
            currString.deleteCharAt(currString.length() - 1);
        }
        if (right < left) { // 能够添加有括号
            currString.append(')');
            generate(currString, left, right + 1, max, ans);
            currString.deleteCharAt(currString.length() - 1);
        }
    }
}

蛮力法

class Solution {
    public List<String> generateParenthesis(int n) {
        List<String> ans = new ArrayList<>();
        if (n <= 0)
            return ans;

        generate(new char[2 * n], 0, ans);

        return ans;
    }

    private void generate(char[] currentString, int pos, List<String> ans) {
        if (pos == currentString.length) {
            if (valid(currentString))
                ans.add(new String(currentString));
        } else {
            currentString[pos] = '(';
            generate(currentString, pos + 1, ans);
            currentString[pos] = ')';
            generate(currentString, pos + 1, ans);
        }
    }

    private boolean valid(char[] currentString) {
        int balance = 0;
        for (char ch : currentString) {
            if (ch == '(') 
                ++balance;
            else 
                --balance;

            if (balance < 0)
                return false;
        } 

        if (balance == 0)
            return true;

        return false;
    }
}